The expression of distinct proteins on the surface of tumor cells offers the opportunity to diagnose and characterize disease by probing the phenotypic identity and biochemical composition and activity of the tumor. Radioactive molecules that selectively bind to specific tumor cell surface proteins allow the use of noninvasive imaging techniques, such as molecular imaging or nuclear medicine, for detecting the presence and quantity of tumor associated proteins, thereby providing vital information related to the diagnosis and extent of disease, prognosis and therapeutic management options. In addition, as radiopharmaceuticals can be prepared that are not only capable of imaging disease but also delivering a therapeutic radionuclide to the diseased tissue, therapy, in particular cancer therapy, can be realized. The selective expression of CA-IX on tumors in response to hypoxia makes it an attractive target to exploit for noninvasive imaging as well as targeted radiotherapy.
It is known that to grow beyond more than a few millimeters in diameter, tumor micrometastasis need to obtain a supply of oxygen to sustain the high metabolic rate characteristic of rapid growth, and do so by inducing the formation of new blood vessels. The distance that tumor cells reside from blood vessels is inversely proportional to the oxygen pressure of the tumor. Even when angiogenesis occurs and a blood supply is established, the less vascular interior region of the growing tumor mass remains hypoxic and eventually undergoes necrosis.
Hypoxia is associated with a poor response to radiation therapy, and leads to tumor resistance. Since oxygen is necessary for the cytotoxic actions of free radicals generated by radiation, higher, often incompatible, levels of radiation are required to promote damage to the tumor. Therefore, there is a need for non-invasive techniques to stratify patients based on cancer hypoxia who are not expected to respond to radiation therapy because of low oxygen, and who may be candidates for alternative hypoxia-activated chemotherapies that are becoming available. As hypoxia constitutes a major difference between the tumor and normal tissues, it can be exploited for the development of tumor specific probes.
Hypoxia is a potent stimulus for the expression of specific genes, several which function to trigger vasculogenesis and therefore supply oxygen to the tumor, increase metabolism to increase the oxygen extraction factor, and promote a favorable environment for tumor growth. The activation of hypoxia inducible genes is in part mediated by a transcription factor, HIF-1α. Under normoxic conditions, HIF-1α is hydroxylated on proline residues that reside in the oxygen induced degradation domain of the protein by proline hydroxylase. Hydroxyproline facilitates binding of Von-Hippel-Lindau Factor (VHL), a tumor suppressor that, when bound, promotes the ubiquitination and degradation of HIF-1α. During hypoxia, proline hydroxylase is inhibited, and VHL no longer binds HIF-1α; the now stabilized HIF-1α translocates to the nucleus and associates with HIF-1α. This heterodimeric transcription factor then binds to HIF-1 responsive DNA sequences in the promoter region of target genes including the carbonic anhydrase isoform CA-IX, as well as VEGF, erythropoietin, and glucose transporters.
Carbonic anhydrases are a family of enzymes comprised of 16 isozymes that catalyze the reaction: CO2+H2OHCO3−+H+, and therefore play an important role in pH regulation. Specific isozymes are found either in the cytosol, anchored to the membrane, within the mitochondria, or secreted from the cell. The well studied constitutively expressed isozyme, carbonic anhydrase II, is found in the cytosol of most cell types, and is the primary isoform responsible for the regulation of intracellular pH.
CA-IX is a membrane-anchored isoform of the enzyme with its catalytic domain in the extracellular space. It has a limited tissue distribution and is found at low levels primarily in the gastrointestinal tract. The expression of CA-IX is under the control of HIF-1α, and this isozyme is highly expressed in tumors cells exposed to hypoxia both in vitro and in vivo. Increased CA-IX expression has been detected in carcinomas of the cervix, ovary, kidney, esophagus, lung, breast, and brain. CA-IX has been reported to promote extracellular acidification. The low extracellular pH as a result of the activity of CA-IX leads to tumorigenic transformation, chromosomal rearrangements, extracellular matrix breakdown, migration and invasion, induction of growth factors, protease activation, and chemoresistance.
CA-IX has been shown by immunohistochemistry and by a variety of molecular techniques to be correlated with tumor progression and poor survival, and has been proposed as a clinical diagnostic and prognostic marker for breast, renal and non-small cell lung cancers. A chimeric 124I-labeled anti-CA-IX antibody G250 is currently undergoing clinical trials for the detection of clear cell renal carcinoma, validating CA-IX as a cancer target.
While intact antibodies such as G250 offer potential for tumor radiotargeting, long circulating half-life and poor tissue penetrability limit their effectiveness as radiodiagnostic and radiotherapeutic agents. A variety of biologically active molecules have been exploited as carriers for the radionuclides. However, small molecules offer significant advantages over antibodies and proteins. In general, the affinity of small molecules for their receptors is similar to that of monoclonal antibodies. Small molecules by definition exhibit enhanced diffusibility to the extravascular space, faster blood clearance resulting in lower background radiation. In addition, the opportunity to synthesize analogs exhibiting diverse chemical properties allows alteration of binding affinity and pharmacokinetics.
Currently, many small molecule inhibitors for CA-IX show undesired side effects due to inhibition of other CA isozymes present in the target organ. Due to the extracellular location of CA-IX, the applicants have discovered that membrane-impermeant CA inhibitors would inhibit selectively only membrane-associated CA isozymes, thus potentially reducing undesired side effects that may arise from inhibition of other, including non-membrane-associated, CA isozymes.